Equalizer containing a plurality of interference correcting equalizer sections

ABSTRACT

An equalizer includes at least two first mutually interfering equalizer sections, and at least two second interference-correcting equalizer sections, arranged in series. Each of the second equalizer sections corresponds with one first equalizer section, such that although each corresponding equalizer has the same center frequency, the second equalizer sections have an equalization response opposite the interference effect, and the gain of the corresponding second equalizer at the respective common center frequency contains the negative gain of at least one first equalizer adjacent to the corresponding first equalizer at the center frequency of the corresponding first equalizer.

PRIORITY INFORMATION

This application is a continuation of Ser. No. 10/081,868 filed Feb. 22,2002 now U.S. Pat. No. 7,145,944.

BACKGROUND OF THE INVENTION

The present invention relates to the field of equalizers, and inparticular to an equalizer that includes a plurality of sections tunedto reduce interference between various sections of the equalizer.

Equalizer arrangements such as equalizer banks comprise a set ofequalizers that are interconnected in a certain way using a serial,parallel, or any other structure. The individual equalizers may bepresence (+) equalizers or absence (−) equalizers, that is, equalizerswith an increase of gain or a decrease of gain (increased attenuation)at the relevant center frequency. The attenuation dimension in thefollowing discussion shall be indicated in dB as a negative gaindimension.

An inherent problem in equalizer banks is the fact that the responsecharacteristic of the individual equalizers within the equalizer bankmutually interferes, creating at least two highly undesirabledisadvantages for conventional equalizer banks. First, depending on thesetting at certain frequencies and frequency ranges, pronounced gainpeaks or attenuation peaks may arise. A second problem is that more orless pronounced distortion of the response characteristic may occur.

Therefore, there is a need for an equalizer that reduces the undesirableinterference between the equalizer sections.

SUMMARY OF THE INVENTION

An equalizer receives an input signal and provides an equalizer outputsignal that includes a first equalizer bank and a second equalizer bank.The first equalizer bank includes a first equalizer section thatreceives the input signal. The first equalizer section has a gain andprovides a first equalizer output signal to a second equalizer sectionhaving a center frequency. The second equalizer section provides asecond equalizer output signal to a third equalizer section that has again and provides a third equalizer output signal. The second equalizerbank includes a first correcting equalizer section that receives asignal indicative of the third equalizer output signal and provides afirst correcting equalizer output signal to a second correctingequalizer section. The second correcting equalizer section provides asecond correcting equalizer output signal to a third correctingequalizer section, which provides the equalizer output signal. Thesecond correcting equalizer section includes a gain value that isindicative of the negative sum of the gains associated with the firstand third equalizer sections of the first bank at the center frequencyof the second equalizer section of the first bank.

An equalizer according to the present invention reduces the amount ofinterference between equalizer sections by taking into account theinterfering effect of at least one adjacent equalizer section,preferably of the two adjacent equalizer sections, at the centerfrequency of the respective equalizer section. The equalizer determinesthe interfering effect of each individual equalizer section on itsadjacent equalizer sections at the center frequency of the latter, andderives correction values from this. Corresponding correction equalizersections with the opposite response are then driven using thesecorrection values to compensate for the interfering effects.

An equalizer arrangement comprises at least two first equalizer sectionsand at least two second equalizer sections (correction equalizers) thatare connected in series. In each case one second equalizer sectioncorresponds with one first equalizer section, such that while thecorresponding first and second equalizer sections have the same centerfrequency, the corresponding second equalizer sections exhibit anequalization response that at least partially compensates for theinterference. The gain of each corresponding second equalizer section atthe relevant common center frequency is equal to the negative sum of thegain of at least one first equalizer section adjacent to thecorresponding first equalizer section at the center frequency of thecorresponding first equalizer section.

The gain of each equalizer section of the equalizer is calculated orestimated at the center frequency of at least one adjacent equalizersection. This gain is then used to adjust the correction equalizersections corresponding to the adjacent equalizer sections. The gainvalues so determined (in dB) may thus be added up with the correct signfor each individual center frequency so as to provide a certain gainvalue section (or attenuation value) for each corresponding equalizer(correction equalizer) at each center frequency. The gain for eachcorresponding second equalizer section (correction equalizer) is equalto the negative interference value calculated for the correspondingfirst equalizer section at its center frequency.

In addition to gain, the phase position at each respective centerfrequency may also be taken into account, which permits a complexcalculation rather than a purely real calculation.

The gain of each second equalizer section is preferably composed of thenegative gains from the first equalizer section preceding thecorresponding first equalizer section and from the first equalizersection following the corresponding first equalizer section at thecenter frequency of the corresponding first equalizer section. Inaddition, the gain of each second (i.e., correction) equalizer sectionmay also contain the negative gains of additional adjacent firstequalizer sections at the center frequency of the corresponding firstequalizer section.

The arrangement of the equalizers may be such that either the firstequalizer sections are each connected immediately adjacent in series orthe second equalizer sections are each connected immediately in series.In an alternative embodiment, first and second equalizer sections may beconnected immediately adjacent in series.

Preferably, the first and second equalizer sections each have anessentially constant specified gain (e.g., the gain 1 corresponding to 0dB) which, however, is raised or lowered relative to the essentiallyconstant value in the center-frequency range. The center frequency mayeither be fixed (graphic equalizer bank) or modifiable (parametricequalizer bank). In the case of parametric equalizers, othercharacteristics of the response characteristic may also be adjustable.

These and other objects, features and advantages of the presentinvention will become more apparent in light of the following detaileddescription of preferred embodiments thereof, as illustrated in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram illustration of a serial equalizer arrangementconfigured as a parametric equalizer;

FIG. 2 is a block diagram illustration of a serial equalizer arrangementconfigured as a graphic equalizer bank;

FIGS. 3A-3C are plots of response characteristics of the equalizersillustrated in FIGS. 1 and 2;

FIGS. 4A, 4B, 5A, 5B, 6A and 6B are plots of the response characteristicof the improved equalizer arrangements in comparison with conventionalequalizer arrangements; and

FIG. 7 is a block diagram illustration of a parallel equalizerarrangement configured as a parametric equalizer.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a block diagram illustration of a serial equalizer arrangement100 configured as a parametric equalizer. The serial equalizer 100includes a first equalizer bank having six equalizer sections 1-6. Thefirst equalizer bank sections 1-6 have center frequencies f₁-f₆,respectively, which establish the response characteristic of the entireserial equalizer arrangement 100. The serial equalizer arrangement 100also includes a second equalizer bank that includes six second equalizersections 7-12. Each of the second equalizer bank sections 7-12 arearranged in serial and have the same center frequency f₁-f₆ of auniquely associated one of the first equalizer bank sections 1-6. Inthis embodiment, the center frequency values f₁-f₆ are variable (e.g.,dynamically adjustable) to provide a parametric equalizer.

Each of the second equalizer bank sections 7-12 has an equalizationresponse that is opposite the interference effects on its correspondingfirst equalizer bank section 1-6 produced by the adjacent firstequalizer bank section. For example, the specific first equalizer banksections 1, 2, 3, 6 are presence (+) equalizers, while the specificfirst equalizer bank sections 4, 5 are absence (−) equalizers. Opposingthese, the specific second equalizer bank sections 8, 10 are presenceequalizers, while the specific second equalizer bank sections 7, 9, 11,12 are absence equalizers, corresponding to the interference effect ofthe first equalizer bank sections adjacent to the corresponding firstequalizer bank sections.

With respect to the base gain, in general the presence equalizersincrease the gain at the relevant center frequency while the absenceequalizers lower the gain at the center frequency. The mode of operationfor absence equalizers and presence equalizers is presented in moredetail in FIGS. 3A-3C.

The gain (+) or attenuation (−) of the second equalizer bank sections7-12 designed as presence equalizers or absence equalizers is derivedfrom the interference (crosstalk) of the first equalizer bank sectionsthat are adjacent to each corresponding first equalizer bank section atthe common center frequency f₁-f₆ of the corresponding first equalizerbank section. In one embodiment, the two first equalizer bank sections,if present, which are each adjacent on both sides of a correspondingfirst equalizer bank section, are evaluated to compute values for thecorresponding second equalizer bank sections 7-12.

In the case of the specific first equalizer bank section 1, there areonly two adjacent first equalizer bank sections, namely, first equalizerbank sections 2 and 3, which are located on the upstream side of firstequalizer bank section 1. Based on its equalization response that isopposite the respective total crosstalk, the second equalizer banksection 7, corresponding to the first equalizer bank section 1, emulatesthe effect of the two adjacent first equalizer bank sections 2 and 3 atthe center frequency f₁ of the two corresponding first and secondequalizer bank sections 1 and 7 in complementary fashion such that theeffect of the two first equalizer bank sections 2 and 3 on the firstequalizer bank section 1 is compensated. The amount of the attenuationof the second equalizer bank section 7 corresponds to the amount of gainat the center frequency f₁, which is produced by the two first equalizerbank sections 2 and 3.

At the center frequency f₃ of the first equalizer section 3, forexample, first equalizer bank sections 1 and 2 also act before the firstequalizer bank section 3, while the first equalizer bank sections 4 and5 do so after the first equalizer bank section 3. The effect of thefirst equalizer bank section 6 that is three places removed from thefirst equalizer bank section 3 is negligible here. Corresponding to thefirst equalizer bank section 3 is the correcting second equalizer banksection 9, which serves to compensate for the effect of the firstequalizer bank sections 1, 2, 4 and 5. The amount of attenuation of thecorrecting second equalizer bank section 9 is equal to the amount ofgain at the common center frequency f₃ produced in common by the firstequalizer bank sections 1, 2, 4 and 5 at the center frequency f₃. Thecompensation at the remaining center frequencies f₂, f₄, f₅ and f₆ iseffected analogously.

The gains or attenuations of the correcting second equalizer banksections 7-12 may be calculated or estimated from the transfer functionof the associated adjacent first equalizer bank sections 1-6, or bymeasurement of the signals produced by the associated adjacent firstequalizer bank sections 1-6 at the respective center frequency.

FIG. 2 is a block diagram illustration of a serial equalizer arrangement200 configured as a graphic equalizer bank. The graphic equalizer bankincludes five first equalizer sections 13-17 as well as five secondequalizer sections 18-22. Each of the first equalizer sections includesa corresponding second equalizer section. The corresponding first andsecond equalizer sections have the same center frequency. Theequalization response of the second equalizer sections is determinedfrom the interference effect at the respective center frequency. Thefirst and second equalizer sections 13-22 are arranged such that thecorresponding first and second equalizer sections immediately followeach other, and the pairs of corresponding equalizer sections 13-22 areconnected in series. Since this embodiment is configured as a graphicequalizer bank, the center frequencies f₇-f₁₁ are fixed. In this case,the second equalizer sections 18-22 may be permanently adjusted to therespective corresponding first equalizer sections 13-17 and theiradjacent first equalizer sections. That is, the second equalizersections 18-22 may also be designed as fixed (preferably identical)equalizers.

In the embodiment of FIG. 2, only the immediately adjacent firstequalizer sections 13-17 are evaluated for interference compensation.For example, the effect of the first equalizer sections 14, 16 adjacentto the first equalizer section 15 may be expressed here formally—asillustrated in the following discussion.

In this embodiment the output variables may include: the respectivegains G₁ (in dB) of the first equalizer sections 13-17, the respectivequality Q_(i) of first equalizer sections 13-17 or the gains G_(k) (indB) of second equalizer sections 18-22 or the respective quality Q_(k)of second equalizer sections 18-22, the respective center frequencyf_(i) of the first equalizer sections 13-17 or the respective centerfrequency f_(k) of second equalizer sections 18-22 as well as the valueK_(ji) (in dB) corresponding to the gain of the first equalizer section13-17 with a center frequency f_(j) at the center frequency f_(i) andthus the crosstalk (interference) of an equalizer section with centerfrequency f_(j) affecting the first equalizer section with centerfrequency f_(i).

The total crosstalk K_(i) from the two adjacent channels j=i+1 and j=i−1can be expressed as:K _(i) =K _((i+1)) +K _((i−1))The gain of the second equalizer section k corresponding to firstequalizer section i is thus:G _(k) =−K _(i)Otherwise the following applies for two corresponding first and secondequalizer sections i and k where f_(i)=f_(k):Q _(k) =α·Q _(i) where 0.5≦α≦2

FIG. 3A is a plot of a characteristic curve of a single first equalizersection located within an equalizer bank, the crosstalk of adjacentequalizer sections being contained in schematic form in thecharacteristic curve shown (gain peaking). FIG. 3B shows thecharacteristic curve for the gain A (in dB) as a function of thefrequency f of a second equalizer section (correction equalizer)corresponding to the first equalizer section shown in FIG. 3A, thecharacteristic curve of this correction equalizer corresponding to theopposite characteristic curve of the crosstalk produced by the adjacentfirst equalizer sections in the corresponding first equalizer section.

Connecting the corresponding first and second equalizer sections inseries results in a total characteristic curve that is shown in FIG. 3C.This characteristic curve matches the adjusted and thus desiredcharacteristic curve since the effect of the adjacent equalizer sectionsis compensated. The characteristic curve is characterized by the centerfrequency f_(i), the quality Q_(i), and the gain G_(i) at the centerfrequency f_(i).

FIGS. 4A, 5A and 6A each show the characteristic curve for an equalizerbank with seven individual equalizer sections without the correspondingequalizers (correction equalizers), while FIGS. 4B, 5B and 6B show thiswith the corresponding equalizer sections (correction equalizers)according to the invention. The following table shows the base settingsin the embodiments of FIGS. 4-6 (the arrows indicate the presetelevations or reductions at the respective center frequencies):

Equalizer 1 2 3 4 5 6 7 Frequency (Hz) 80 200 500 1k 2k 5k 12k Gain (dB)+10 +10 +10 +10 +10 +10 +10 In FIG. 4 Gain (dB) 0 0 −10 +10 0 0 0 InFIG. 5 Gain (dB) +10 +10 +10 −10 +10 +10 +10 In FIG. 6Significant reductions are shown in FIG. 4 for undesired gain peaking,in FIG. 5 for undesired gain reduction, and in FIG. 6 for undesireddistortions of the response characteristic. Only the two immediatelyadjacent equalizer sections are being evaluated, however, in FIGS. 4-6.As a result, peaks of up to 15 dB were compensated.

In the equalizer arrangements shown, methods are employed in which afirst equalization of the input signal occurs at, at least two centerfrequencies with a certain equalization response, and a correspondingsecond equalization of the input signal occurs at the same at least twocenter frequencies with a corresponding equalization responsecompensating interference, where the gain of the respectivecorresponding equalizations (correction equalizations) at the respectivecommon center frequency contains the negative gain of at least one ofthe first equalizations adjacent to the corresponding first equalizationat the center frequency of the corresponding first equalization. Amethod of this type may be preferably employed either in an analog ordigital signal processor.

FIG. 7 is a block diagram illustration of a parallel equalizerarrangement 700 configured as a parametric equalizer. A path of constantequalization is maintained from input to output. The gain is equal toone (or 0 dB) in this embodiment. However, it may also have any othervalue for gain/attenuation. First equalizer sections 23-27 and secondequalizer sections 28-32 are preferably bandpass filters with highattenuation below a lower cut-off frequency, high attenuation above anupper cut-off frequency, and variable gain at the center frequency.

The first equalizer sections 23-32 are interconnected on the input sidein parallel and coupled with the input of the equalizer arrangement 700.On the output side, all the first equalizer sections 23-32 lead to asummer 33, which provides an equalizer output signal. Connected inseries to the second equalizers sections 28-32 are controllableamplifiers 34-38, respectively.

The individual first equalizer section 23-27 may interfere in complexfashion (i.e., the intensity of the interference is a function of theincoming phases). Each first equalizer section corresponds with anassociated one of the second equalizer sections 28-32 correcting theinterference, such that they have the same center frequency. The totalinterference effect of adjacent equalizer sections at the centerfrequency of an individual first equalizer section may be calculated orestimated either simply as a real number (quantity) at approximately thesame phase position, or more precisely as a complex number (quantity andphase).

To reduce interference by adjusting the (real) gain of each correctingthe second equalizer sections 28-32 in a simple manner withoutconsidering phase, the determined gain quantity (real value) issystematically varied as required by the controllable amplifiers 34-38,while the output of each second equalizer section is provided asrequired with a reverse sign (phase reversal). This approach takes intoaccount, to a certain extent, the phase relationship that was ignoredfor the sake of simplicity.

One of ordinary skill in the art will recognize that although thepresent invention has been discussed in the context of certain numbersof first and second equalizer sections, of course the present inventionis not so limited. For example, the present invention is not limited toan equalizer having seven first equalizer sections and seven secondequalizer sections.

Although the present invention has been shown and described with respectto several preferred embodiments thereof, various changes, omissions andadditions to the form and detail thereof, may be made therein, withoutdeparting from the spirit and scope of the invention.

1. An equalizer arrangement for generating an output signal byequalizing an input signal, comprising: a first equalizer section havinga first center frequency f1 that receives the input signal and providesa first equalizer output signal to a second equalizer section having asecond center frequency f2 that provides a second equalizer outputsignal; a first correcting equalizer section that receives the secondequalizer output signal and has a gain at the first center frequency f1equal to the gain of the second equalizer section at the first centerfrequency f1 to provide a first correcting equalizer output signal; anda second correcting equalizer section that receives the first correctingequalizer output signal and has a gain at the second center frequencyvalue f2 equal to the gain of the first equalizer section at the secondcenter frequency f2 to provide the output signal, where f1 and f2 aredifferent values.
 2. An equalizer that receives an input signal,comprising: a first equalizer, including a first equalizer sectionhaving a gain G1 at a first center frequency f1, that receives the inputsignal and provides a first equalizer output signal to a secondequalizer section having a gain G2 at a second center frequency f2,where the second equalizer section provides a second equalizer outputsignal to a third equalizer section having a gain G3 at a third centerfrequency f3 that provides a third equalizer output signal; and a secondequalizer having a first correcting equalizer section that receives asignal indicative of the third equalizer output signal and provides afirst correcting equalizer output signal to a second correctingequalizer section that provides a second correcting equalizer outputsignal to a third correcting equalizer output section, which provides athird correcting equalizer output signal, where the second correctingequalizer section includes a gain value that is indicative of thenegative sum of the gains associated with the first and third equalizersections at the second center frequency f2.
 3. The equalizer of claim 2,where each of the sections of the first and second equalizers includes abandpass filter.
 4. An equalizer that receives an input signal,comprising: a first equalizer bank, including a first equalizer sectionhaving a gain and that receives the input signal and provides a firstequalizer output signal to a second equalizer section having a centerfrequency f2, where the second equalizer section provides a secondequalizer output signal, and a third equalizer section that receives thesecond equalizer output signal and provides a third equalizer outputsignal; and a second equalizer bank, including a first correctingequalizer section that receives a signal indicative of the thirdequalizer output signal and provides a first correcting equalizer outputsignal to a second correcting equalizer section that provides a secondcorrecting equalizer output signal to a third correcting equalizersection that provides an equalizer output signal, where the secondcorrecting equalizer section includes a correction gain value at thecenter frequency f2 that is indicative of the negative sum of (i) thegain of the first equalizer section at the center frequency f2 and (ii)the gain of third equalizer section at the center frequency f2.